Vertebrae in a spinal column are separated from one another by intervertebral discs that cushion the vertebrae and enable the spinal column to bend and twist. Mechanical stress and disease may cause damage to discs and vertebrae, possibly reducing intervertebral spacing, causing back pain, and reducing a range of motion for the spine. Pain, and possibly neurologic injury, may be caused by bone spurs forming on vertebral surfaces, by disc material herniating into the spinal canal, or for other reasons.
Surgical treatment to relieve pain and restore patient mobility may include removal of spurs, removal of discs, and fusion of adjacent vertebrae. Vertebrae may be fused together to prevent relative motion between the vertebrae, set intervertebral spacing to a preferred value, restore the spine to a preferred curvature, and for other reasons. Fusion may be achieved by connecting the vertebrae to one another with plates, screws, or spacing devices referred to as cages, and by procedures to encourage bone to grow between and join together adjacent vertebrae. Graft material may be placed into intervertebral spaces to enhance fusion. A cage to be inserted into an intervertebral space may be formed with an aperture for holding graft material in place while bone grows between vertebrae. Cages and other hardware may be made from titanium, stainless steel, polyether ether ketone (PEEK), graphite, carbon fiber, or other strong, stiff materials capable of withstanding the mechanical stresses and chemical and biological agents found in a spinal column.
Cages and other devices may be selected to securely hold adjacent vertebrae at a preferred intervertebral separation distance until fusion is achieved. For example, a cage shape and size may be selected to restore intervertebral spacing to a condition close to the spacing that existed when two adjacent vertebrae were separated by a healthy, nondegenerate disc. The optimum shape and size of a cage may vary with position along a spinal column and from patient to patient. However, it may be difficult to match the size and shape of a cage to a specific spinal condition. A cage that is too small may migrate away from an intervertebral position selected during surgery or may interfere with the progress of bone growth for fusion. A cage that is too large may be difficult to insert in to an intervertebral space or may subside into adjacent vertebrae. A cage that is the wrong size may fail to alleviate pain or may lead to damage to bone or neurological tissue. Selecting a size and shape of cage that will neither migrate nor subside after surgery may require repeated medical imaging of the contact area between cage and vertebrae during surgery, and may require replacement of one cage with another of a different size or shape until a preferred intervertebral spacing and preferred contact between cage and bone is determined by a surgeon.